On the origin of 3-methyl steroids and diasteranes

Abstract

The potential origin of 3-methyl steroids is investigated through comparison of their carbon number distributions with those of their desmethyl counterparts using GC-MS and GC-MS-MS analysis of a range of oils and some immature source rock bitumens. There is a close genetic link between desmethyl and 3-methyl steranes, consistent with competition between methylation and hydrogenation at C-3 in the shared steroidal precursors, producing strongly positively correlated carbon number distributions within the two series. The absence (or possibly very low levels) of detectable 3-methyl diasteroids in immature bitumen samples suggests that these compounds are not formed in the free bitumen during diagenesis in amounts capable of explaining their abundance in oils. It is possible that they are produced by a limited degree of rearrangement of steroids after they are bound into kerogen or upon cleavage from kerogen and methylation during catagenesis. Support for this proposition is provided by the previous detection of desmethyl diasterenes in the hydropyrolysate of pre-extracted algal biopolymers, as well as the high diasterane/sterane ratios reported in several studies of hydrous pyrolysis of kerogen. Competition between methylation and hydrogenation during catagenetic formation of diasteranes might explain the lower diasterane/regular sterane ratio observed for 3-methyl compared to desmethyl species.